MERLIN is a Franco-German collaborative minisatellite climate mission. The primary objective is to obtain spatial and temporal gradients of atmospheric methane (CH4) columns with high precision and unprecedented accuracy on a global scale. Methane (CH4) and carbon dioxide (CO2) both cause global warming, although the impact of methane is 25 times more powerful than the one of carbon dioxide on a timescale of 100 years. Now, at a time when there is much discussion about mankind being directly responsible for the rise in the emission of greenhouse gases, methane emission levels already far outstrip carbon dioxide. Since pre-industrial times, the amount of methane in the atmosphere has more than doubled, whereas the growth in carbon dioxide levels during the same period has been 'only' thirty percent. Methane is after carbon dioxide the strongest anthropogenic greenhouse gas. Methane emissions are caused by human activities as well as natural sources – for example, from rice paddies, animal husbandry, biomass decomposition, landfill sites or energy generation. Natural sources include swamps and marshlands as well as thawing permafrost. Alongside carbon dioxide, methane is one of those gases for which the Kyoto Protocol stipulates that cuts must be achieved. 1)2)3)4)5)

A major problem in the understanding of CH4 source- and sink-processes is the lack of precise global measurements of atmospheric CH4. Ground based in-situ observations are insufficient because the existing measurement network is too coarse. Source regions of key importance to the global CH4 cycle such as the Arctic permafrost, Boreal forests and Tropical wetlands are difficult to access; hence, they are underrepresented or not sampled at all. Therefore, it is necessary to apply spaceborne measurement techniques in order to obtain global coverage at high precision. Today, GOSAT (Greenhouse Gases Observing Satellite) of JAXA has the ability to measure CH4 from space. The observation strategy is based on measuring spectra of sunlight backscattered by the Earth's surface and atmosphere in the shortwave infrared spectral region. The main problem of these passive methods is that undetected aerosol layers or thin ice clouds produce systematic measurement errors of unknown magnitude, resulting from the complexity of the retrieval algorithms and the limited availability of independent measurements for validation. To counter these limitations, the use of active remote sensing instruments like the IPDA (Integrated Path Differential Absorption) LIDAR, was proposed.

The data that the MERLIN climate satellite will gather from orbit will enable scientists in both countries to draw conclusions about the various different sources of methane emissions. What is the impact of rising levels of energy production? What are the implications when tracts of permafrost release methane as they start to thaw? Above all, what are the implications for our climate?

MERLIN is a joint mission by DLR (German Space Administration) and CNES (French Space Agency), where Germany is developing and building the methane LIDAR (Light Detection And Ranging) instrument, while France is providing the satellite platform Myriade Evolutions and the mission control. Joint data processing and science activities in France and Germany will be established.

The sharing of the Parties' undertakings schemed for the overall MERLIN project is as follows:

• CNES, as system and satellite prime:

- leads system and satellite level activities, including the system architecture definition and the satellite design based on MYRIADE Evolutions platform

- manages the launcher activities based on a preference on European operators

- is in charge of the satellite CGS (Control Ground Segment) based on the new product line, the so called ISIS (Initiative for Space Innovation Standard); this CGS using the CNES earth terminal network

- is responsible, on the PLGS (Payload Ground Segment) topic, for the development of the host structure, in which the operational data processing are implemented, and its exploitation, and the development and the maintenance of the operational level 0, 2 and 3 operational data processors.

• DLR, as payload prime:

- leads payload level activities and some system level activities, including payload development, until its integration on the satellite and its functional verification

- is in charge of the development, the maintenance, and the exploitation of the PLOC (Payload Operation Center) which consists in commanding, monitoring and expertising the payload

- is responsible, in the PLGS scope, of the development and the maintenance of the operational level 1a and 1b operational data processors.

The science activities are led by two Co-Principle-Investigators from the French LMD (Laboratoire de Météorologie Dynamique) of CNRS and the German DLR Institute for Atmospheric Physics, with additional support of several French and German Research Institutes.

The MERLIN mission was initiated in February 2010, when the Franco-German Council of Ministers decided to start a minisatellite mission monitoring the greenhouse gas methane in the atmosphere. Phase 0 of the project started in early 2010. In the second quarter of 2012, MERLIN successfully finished Phase A. A launch is expected in the timeframe of 2021 with a minimum mission life time of 3 years on orbit. 6)7)8)9)

Table 2: Comparison of the MERLIN mission with other past, current, decided, or proposed missions measuring methane with characteristics that are suitable for source estimation. 10)

CNES is providing the MERLIN platform, a minisatellite of the Myriade platform series, referred to as "Myriade Evolutions". The Myriade Evolutions program is an enhancement of the original platform concept in order to improve the payload allocations in terms of power, mass, volume and reliability. In fact, the main improvements are: a new structure in order to reach 400 kg for satellite mass, an increase of the solar array capacity and the power distribution, a new adapted propulsion system, an improved AOCS (Attitude and Orbital Control Systems), an increase of the payload data storage, the obsolescence handling, a compatibility with low flight altitudes (atomic oxygen concerns).

Initiated by CNES, this program is developed in partnership with Thales Alenia Space and Airbus Defence and Space, France. The first Myriade Evolutions platform is implemented for the MERLIN project. MERLIN aims at developing and qualifying the platform product line which will be available for the time frame 2015-2025. This product line will provide platforms for scientific, defence or industrial applications. As it has been the case for the previous product line MYRIADE, a low cost, a modularity which fits the mission needs and a short development schedule are the main assets of the Myriade Evolutions Program. 11)12)13)14)15)16)

The Myriade Evolutions platform is a standard platform composed of a rectangular parallelepiped structure set up of:

• A Spacecraft Interface Ring which ensures the interface between the launcher and the satellite

The chemical propulsion module is bi-compliant with green propellant and hydrazine. The module, based on 4 thrusters 1N allows the positioning operations, in orbit operation and deorbiting at end-of-life phase. The power subsystem is composed of a 2 wings solar generator, a battery and a new equipment for the electrical conditioning and the distribution. Those elements are completed by 4 reaction wheels, 2 GNSS (GPS), 2 S-band transmitters for the telemetry (1 Mbit/s) and the telecommands (32kbit/s), a Payload Mass Memory and an X-band transmitter high rate (180 Mbit/s) for the science data. The equipment developed in the frame of Myriade Evolutions are aimed to equip the platform with efficient and reliable product.

The avionic cores (on board computer (OBC) and attitude and orbit control system (AOCS)) are based on the avionics upgraded from the existing product lines of each satellite contractor (ADS-F and TAS) in order to maximise the synergy between the low orbit satellite family. This choice ensures to both contractor a high knowledge and control of the product line.

To sum up, the Myriade Evolutions Platform is composed of a structure and 9 common units which are completed by the avionics and the AOCS equipment provided by the satellite contractor. Moreover, all those developments are conducted in conformity with the FSOA (French Space Operations Act) with embedded functions as end of life passivation (tank unpressured actions and electrical passivation).

In April 2015, Airbus Defence and Space signed a contract with CNES to build the platform and carry out final integration of the MERLIN spacecraft. MERLIN is the first institutional program to be based on the new AstroBus-S platform, developed by Airbus Defence and Space in partnership with CNES, as part of the French Investment for the Future Program (Plan d'Investissements d'Avenir – Myriade Evolutions). AstroBus-S will form the basis of the Airbus Defence and Space range of 400 kg class Earth observation satellites for imaging, climatology and science missions. 17)18)

The Myriade Evolutions platform is (Figure 2) is not compatible anymore with the Soyuz outer seat position. The satellite volume is constrained by the Soyuz inner seat position which is the most limiting of the anticipated potential launchers (European and non-European).

• Units belonging to the so-named "common units", specified, developed and qualified by the Myriade Evolutions partnership (CNES, Airbus Defence and Space and Thales Alenia Space)

• Units belonging to the so-named "avionic core units", specified, developed and qualified by the satellite prime contractors (Airbus Defence and Space and Thales Alenia Space).

• MERLIN is the first satellite application of Myriade Evolutions and the second instantiation of the AstroBus-S platform.

Of course, the common units and the avionic core units are designed in order to guarantee a trans-compatibility in terms of functions and interfaces.

The common units are specified and selected further to a European consultation. The avionics core is a heritage of the multi-mission version of avionics developed in the recent past by the French prime space industry (Airbus Defence and Space and Thales Alenia Space). These avionic cores, compliant to ISIS standard, ensure a high level of modularity with the payload interfaces.

The MERLIN satellite architecture is then based on the AS250 (AstroSat) avionics core and integrates all Myriade Evolutions common units, as shown in Figure 4.

AOCS (Attitude and Orbit Control Subsystem): The AOCS, selected for MERLIN mission, is the high-accuracy gyroless version, fully inherited from Airbus Defence and Space flight proven AS250 avionics (flown on SPOT-6 and SPOT-7) and based on a dual head Hydra star tracker. The equipment (actuators) coming from Myriade Evolutions are integrated in the AS250 AOCS with only minor changes at the level of major functional interfaces.

All other functional chains (electrical power, S-band, payload data handling and transmission, propulsion) are based on Myriade Evolutions units, and fulfil all the needs and performance requirements of the MERLIN mission. The resulting evolution of AstroBus S platform will be qualified for the MERLIN satellite, first applicative mission selected by CNES for the Myriade Evolutions program.

The redundancy architecture can be tuned to meet mission requirements. For stringent missions, full redundancy can be accommodated. For the MERLIN case, a specific redundancy scheme is selected to meet CNES requirements. This limited redundancy scheme allows exceeding the MERLIN reliability requirement of 0.8 over the lifetime in orbit.

The Central Software architecture is based on a standard structure breakdown with a very high level of reuse from the AS250 product line. Thanks to this standardized architecture, the mission customization is eased and affects only the higher application level.

RF communications: The TT&C data are transmitted in S-band at 625 kbit/s. The payload data are downlinked in X-band at 180 Mbit/s.

Development status of the MERLIN project:

• June 2017: MERLIN shall be the first active mission in space dedicated to GHG (Greenhouse Gases). MERLIN will provide truly global and high accuracy measurements of XCH4 to estimate CH4 fluxes at regional to continental scale. 20)

• On February 17, 2017, DLR and Airbus DS GmbH signed a contract for the design and construction phases of the German-French climate satellite MERLIN (Methane Remote Sensing LIDAR Mission). From 2021, this small satellite mission will measure the methane concentration in Earth's atmosphere to an unprecedented level of accuracy and thus contribute to research into the causes of climate change. 21)

- The contract was signed at the Airbus site in Ottobrunn and includes the German contribution to the mission – namely, the development and construction of the methane LIDAR (Light Detection And Ranging), the measuring instrument on board the MERLIN satellite. The core part of the instrument is a laser, which can send out light pulses on two different wavelengths and thus measure the methane concentration at all latitudes with great precision regardless of sunlight.

• December 08, 2015: At the 2015 UN Climate Change Conference COP 21 in Paris, the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) and the French space agency (CNES) met to reaffirm their commitment to jointly develop the MERLIN (MEthane Remote sensing LIdar missioN) satellite that is set to measure concentrations of methane in Earth's atmosphere with unprecedented accuracy. 22)

• The MERLIN project is currently (2014) in phase B: preliminary design. The MERLIN feasibility was demonstrated in the phase A, it was shown that it is possible to operate a complex IPDA LIDAR instrument on a Myriade Evolutions small satellite platform. Thanks to the new Myriade Evolutions capacities, the payload needs in terms of power, volume, mass, pointing are met. In its current configuration, MERLIN reaches the scientific threshold requirements for the DAOD and is thus compliant with the demands of the scientific USR (User Requirement Document). The major milestones foreseen for the rest of the development are listed in Table 5.

Orbit definition: An important aspect in the mission design is the orbit selection. For MERLIN, a near-polar SSO (Sun-Synchronous Orbit) with an orbit height of about 500 km, a LTAN (Local Time Ascending Node) of 06:00 or 18:00 hours (as shown in Figure 9) and a repeat cycle of about 28 days is foreseen. 23)

Due to a potential late LTAN choice, the satellite (platform and payload) must be designed to be compatible with both orbits at 06:00 and at 18:00 hours; this aspect is a main driver for satellite definition. Because of the low altitude of the mission, a large amount of fuel is dedicated to semi major axis maintenance. This propellant need highly depends on the solar activity. A large uncertainty exists in the prediction of the future solar activity. Sizing and typical predictions are used for fuel budget estimation.

Sensor complement: (IPDA LIDAR)

IPDA (Integrated Path Differential-Absorption) LIDAR:

Measurement principle: An IPDA LIDAR uses the laser light scattered back from a surface to obtain measurements of the column content of a specific atmospheric trace gas between instrument and scattering surface. For this, the difference in atmospheric transmission between a laser emission with a wavelength placed at or near the center of a CH4 absorption line (λon) and a reference wavelength (λoff) with significantly less absorption is used. A telescope collects the backscattered photons and focuses them onto the detector. Since the return signals are very weak, it is necessary to accumulate several single measurements of the return signals along the track in order to achieve the required measurement sensitivity. From the ratio of the two return signals, the DAOD (Differential Atmospheric Optical Depth) can be calculated. 24)25)26)

The goal of the MERLIN mission is to measure the spatial and temporal gradients of atmospheric CH4 columns with high precision and unprecedented accuracy. The main data product of MERLIN will be column-weighted dry-air mixing ratios of CH4, (XCH4) measured along the subsatellite track according to equation 1:

with the received signal powers Poff and Pon, normalized by the associated ratio of transmitted pulse energies Eon and Eoff. Psurf is the surface pressure at the location where the laser beam hits the ground and WF is the weighting function describing the altitude sensitivity of XCH4.

In order to provide a solid scientific basis for the mission, the scientific performance requirements of were formulated. They are based on the random and systematic error of the instrument (Table 6). The requirements were chosen in such a way that the following quality levels can be reached: to resolve large wetland fluxes, inter-hemisphere gradients, seasonal and annual budgets on continental scale (threshold), to resolve the seasonal and annual budgets on country-scale (breakthrough ), highest Methane flux estimate quality, and the Kyoto protocol like monitoring (goal).

The DLR instrument selected consists of an IPDA LIDAR. The industrial consortium, comprising Airbus DS GmbH (former Astrium GmbH, Ottobrunn) and Kayser-Threde GmbH, is realizing this LIDAR instrument, where Astrium is acting as the prime leader and is contributing the transmitter laser. Kayser-Threde is responsible for the optical receiver system and the signal chain. 27)28)29)

Main driver for the actual payload design are the limited payload allocations by the platform. The Myriade Evolutions platform provides a payload allocated power of about 110 W (during eclipse phase) and can carry a payload with a maximum mass of about 95 kg in a volume of 82 cm x 88 cm x 92 cm.

Aside from these platform constraints, the main driver for the instrument design is the necessity to reach the scientific requirements. The RRE (Relative Random Error) is mainly driven by orbit altitude, size of the receiver telescope and the available laser pulse energy. The minimal orbit altitude is limited by the platform (due to atomic oxygen constraints), maximum telescope size due to volume allocations of the launcher and the pulse energy due to power allocations by the platform. Aside from the random error, the RSE (Relative Systematic Error) has also to be taken into account. This error depends mainly on the accuracy and stability of the energy calibration, stability and knowledge of the laser frequency, stability and knowledge of pointing and detector linearity.

The industrial consortium, comprising Astrium GmbH and Kayser-Threde GmbH, is realizing this Lidar instrument, where Astrium is acting as the prime leader and is contributing with the transmitter laser. Kayser-Threde is responsible for the optical receiver system and the signal chain. 30)31)32)

The IPDA LIDAR instrument uses two separate telescopes for the transmission (TX) and reception (RX) of the laser. Both telescopes are mounted on the same optical bench, which accommodates also the laser source unit, the detector chain as well as two star trackers for verification and control of the pointing.

The basic scheme of the instrument is shown is Figure 10. Its main components are (Ref. 11):

• Instrument control unit (a)

• Laser transmitter (b)

• FRU (Frequency Reference Unit) (c)

• Energy calibration unit (d)

• Receiver chain (e)

• Receiving telescope (f)

• Transmitter telescope (g)

One of the main challenges of the MERLIN mission is to implement a complex LIDAR instrument on a small satellite platform with limited resources. MERLIN will be the first satellite-based IPDA LIDAR system. Important aspects for the selection of the subsystems are: power consumption, mass and volume allocations, thermal behavior, and radiation hardness. The main parameters of the MERLIN instrument can be found in Table 2.

The main structural element of the payload is the optical bench, made of UHM (Ultra High Modulus) CFRP (Carbon Fiber Reinforced Plastic) and designed for maximum stiffness and lowest CTE (Coefficient of Thermal Expansion), and sporting a tower for the M2 mirror of the RX telescope. Primary and secondary mirrors of both TX and RX telescope are made of Zerodur (Ref. 28).

The aim of the optical bench is twofold: on one hand it is meant to ensure a proper operational alignment of the two telescopes with the start tracker, on the other hand it thermally and thermoelastically decouples the payload from platform for greater stability.

The choice of a design using UHM CFRP with epoxy resin, coupled with Zerodur mirrors, has been made based on uncertainties on the operational thermal environment (both in terms of absolute temperature and gradients), which are strictly related to the choice of a totally passive thermal control for the payload. In fact, the implementation of an active control would have been hardly feasible due to limitations in the power available to the payload. Of course, this does not exclude that other solutions, e.g. based on silicon carbide, would have been possible too: in fact, within a DLR contract, the involvement of a German material supplier (i.e. Schott AG) for Zerodur represents an additional programmatic advantage on top of technical considerations.

The optical bench is connected to the bus via three isostatic mounts: two long mounts in –z direction, and one short support in +z direction, providing the needed thermoelastic and thermal decoupling. The design of the isostatic mounts is stiff in axial and tangential direction; their orientation and accommodation does not follows a classic 120º spacing, but is trimmed mainly to satisfy the interface requirements imposed by the platform, still avoiding conflicts with other payload elements or the use of asymmetric configurations. In particular, it is highly desirable to place all mechanical interfaces between payload and platform as close as possible to the edges of the platform top floor, since such locations benefit from the stiffening of the platform side panels.

The slanted position of the baseplate matches the angle of the TX-M1 mirror, which has a constant thickness and a flat backside for easier manufacturing and verification. The orientation of the baseplate enables the use of identical isostatic mounts for the TX-M1 mirror, with considerable advantages from the thermoelastic point of view (no rotations of the mirror in case due of global temperature changes).

The receiver telescope uses an afocal design with magnification 50 x. It consists of two conical mirrors and an achromatized collimator lens, which generates an image the entrance pupil with a size of 13.8 mm x 74 mm behind the lens. A design driver has been a compact envelope with a maximal M1-M2 mirror distance of 470 mm, allowing the M2 partly to vignette the entrance pupil. The telescope, together with the collimating lens group has been designed for both 1.645 µm and 633 nm, in order to allow operation and alignment in the visible wavelength range.

The RX-M2 tower consists of a stiff sandwich with CFRP skins. It is directly connected to the optical bench via dedicated bonded brackets. The shape is determined as a best compromise between stiffness, thermoelastic performance and ease of manufacturing. The baffle of the RX telescope is a thin structure out of a light material, e.g., a CFRP laminate. While no stringent requirements on the eigenfrequency of this structure apply, due to the limited mass, stiffening rings are planned to keep a correct shape and avoid strength issues e.g. during acoustic tests. As the optical system is relatively insensitive to straylight, the baffle has mainly a thermal function. The shape of the baffle closely follows the RX beam to maximize such function.

TX telescope: The RX chain clearly dominates the design of the payload due to the larger complexity and physical dimensions. Most of the work has been therefore focused on the RX path, in order to soon identify critical aspects in terms of feasibility or system performance. Nevertheless, the study of the TX telescope can not be neglected. It consists of a separate structure with a ~220 mm M1 mirror; a CFRP baffle is used also as structural element and supports the TX-M2. In this way, the baffle lends the spacer function between M1 and M2 and no additional structures are used, with positive effects on the resulting total mass.

The CFRP baffle and the TX-M1 mirror are supported by an Invar ring-shaped component that is directly connected to the optical bench. The Invar ring lends interfaces for the mounting of the Active Pointing Control mechanism, which folds the laser beam from the laser to the TX-M2 mirror. - Similarly to what done for the RX-chain, the CFRP baffle sports a slanted cut in order to avoid sun illumination into the telescope during nominal operation.

The remaining elements of the payload [radiators, FRU (Frequency Reference Unit), Laser Electronics and ICU (Instrument Control Unit)], as well as the antenna tower, are mechanically interfaced and supported by the platform. The FRU and the ICU, together with the laser, are the main dissipating sources and are thermally coupled with the payload radiators. 33)

The functional MERLIN payload architecture is illustrated in Figure 14 and consists of three main functional parts: The receiver chain, including the receiver (RX) telescope, internal power calibration and the detection unit, the transmitter chain, including the transmitter (TX) telescope, the laser head and laser electronics, the frequency reference unit, and the ICU (Instrument Control Unit).

Laser: As experiences and problems encountered in other missions like EarthCARE (Earth Clouds, Aerosols and Radiation Explorer) and ADM-Aeolus (Atmospheric Dynamics Mission) demonstrate, the design and realization of spaceborne lasers is rather sophisticated and involves several risks which have to be taken into account. These risks are mainly contributed to stable operation under space conditions and lifetime aspects of components. In contrast to the ALADIN instrument (Atmospheric Laser Doppler LIDAR Instrument) of the ADM-Aeolus mission, the laser used for the MERLIN mission is not a completely new development but deduced from the ESA EQM (Engineering Qualification Model) FULAS (Future LAser System) which is currently under construction.

The first step in the laser design was the selection of the appropriate measurement wavelength which depends mainly on these factors: CH4 absorption bands, detector efficiency and eye safety considerations. In the SWIR (Short-Wave Infrared) spectral range, where eye safety considerations are less critical, the CH4 lines are abundant, but the presence of water vapor and carbon dioxide lines drastically constrains the selection. Essentially two atmospheric water vapor transmission windows around 1600 and 2300 nm allow CH4 measurements. The detector performance is significantly better in the 1600 nm region where low-noise photodiodes (InGaAs APDs) are available. Since no directly pumped laser at this wavelength is available up until now, or its technology level is too low for consideration, a laser concept based on a Nd:YAG pumped OPO (Optical Parametric Oscillator) was selected.

The laser concept is based on a two stage concept comprising a single frequency high power Nd:YAG master oscillator power amplifier (MOPA) stage which generates ns pulses at a wavelength of 1064 nm (Figure 15). The temporal, spectral and geometrical properties of the pump beam at 1064 nm are generated in a low power frequency stabilized oscillator. This beam is amplified in the subsequent amplifier stage which optimally only increases the pulse energy without changing the other properties. The output is then converted into the final wavelength of 1645 nm in a second stage using a nonlinear converter. The most established technology for the conversion is an OPO. This architecture has been demonstrated by different groups in various flight campaigns.

The pump laser will consist of a seeded oscillator followed by a single-end pumped slab amplifier. The oscillator design includes an end-pumped rod-crystal as gain medium. The oscillator will be injection seeded and cavity controlled in order to achieve single longitudinal mode operation and to fulfill the stringent requirements on the pulse quality. For the amplifier, the so called InnoSlab concept, developed by the Fraunhofer Institute for Laser Technology, will be used. For the amplifier, the so called InnoSlab concept, developed by the Fraunhofer Institute for Laser Technology, will be used (Figure 16). This concept is ideal for high energy and high efficiency power amplifiers for single mode operation. For MERLIN, the slab crystal is partially end pumped from one end. This gives the advantage that only two faces of the crystal need to be optically polished. The heat dissipates very efficiently and homogeneously over 2 metal heat sinks soldered to the large faces of the crystal. The four optically unused surfaces of the crystal are roughened to optimally prevent internal parasitic laser oscillations.

The four optically unused surfaces of the crystal are roughened to optimally prevent internal parasitic laser oscillations. To operate the InnoSlab laser as an amplifier (right part of Figure 16), the signal beam is folded in a single pass configuration through the crystal. By choosing appropriate mirror radii and signal beam divergence, the beam is widened with every pass, the fluency can be kept constant and remains far away from the damage threshold. By adapting the slab crystal width and the number of passes the InnoSlab amplifier is scalable in its output power. The MERLIN amplifier will be designed to deliver an output pulse energy of about 30 mJ at 1064 nm while the peak fluency will be smaller than 2.5 J/cm2. Simulations showed that this output energy is the best compromise between strain of the pump modules and efficiency of the pumped OPO.

The baseline for the OPO is a 4-mirror 2-crystal setup. The same setup is also implemented in the DLR airborne demonstrator for the DLR Jet HALO (High Altitude and Long Range Research Aircraft). The crystal will be either KAT (Potassium Titanyle Arsenate) or KTP (Potassium Titanium Oxide Phosphate). Like the oscillator, the OPO will be injection seeded and cavity controlled in order to achieve single longitudinal mode operation.

The practical realization of the MERLIN laser transmitter is based on the results of the FULAS (FUture LASer) project that has been initiated by ESA, in cooperation with DLR, to develop and built a technology demonstrator and to verify its suitability for potential space missions. In order to cover the common need of possible future LIDAR missions, requirements for a generic laser source had been defined to achieve maximum usability of the laser concept and technology for future LIDAR missions. 34)

FULAS is built as a pressurized and hermetically sealed housing with an isostatic mounting I/F for accommodation. As shown in Figure 17, the housing central main frame provides several hermetical feedthroughs for electrical and optical connectors, thermal-hydraulic feedthroughs for miniature LHPs (Loop Heat Pipes) and a beam exit window. An external cold plate attached to the central frame of the housing serves as condenser for the LHPs and as overall system thermal I/F. The mounting of the laser optical bench is optimized for decoupling from the surrounding structure. By isostatic mounting inside the housing central frame, the optical bench is insulated from stress induced by the instrument environment. Particularly it features a maximum tolerance with respect to mechanical deformation due to environmental pressure changes.

Due to the need of a lightweight design while providing the thermal conductivity to support thermal balancing, aluminum was the material of choice for the housing. To provide access to both sides of the double sided optical bench, a symmetrical open frame is used, carrying the optical bench. This frame is closed by two symmetrical bolted covers, primary-sealed by special metallic gaskets, enabling easy reopening and resealing during assembly and testing. For final life-time sealing, the covers are designed to be welded, providing a reliable redundant secondary sealing.

The challenging part of the design is the need of a multitude of various feedthroughs (electrical, thermal and fiber-optical) to operate such a system, as well as a high quality optical beam exit window. To fulfil the cleanliness and lifetime requirements, only thermal joining methods as welding, brazing or glass/ceramic potting are applicable. For brittle materials like glass a CTE-matched joint is necessary, which typically leads to low CTE metals like stainless steel or titanium, not compatible with an aluminum housing and conventional joining technology. However, the applied mechanical friction stir welding (FSW) process, developed by the Airbus Group in the frame of the FULAS project, overcomes this incompatibility of the materials.

To allow an efficient rejection of the thermal load inside the pressurized housing and minimize thermal gradients across the optical bench, mini LHPs are directly attached to units inside the laser housing. Four pairs, each operated in one-out-of-two cold-redundancy configuration, are connected with their evaporators directly to the main heat sources on the optical bench, enabling selective cooling directly at the source. By its semi-rigid tubing combined with dedicated housing feedthroughs, the heat is efficiently transported out of the housing towards the external cold plate. Due to the low mechanical stiffness of the tubes the optical alignment and the mechanical decoupling of the optical bench is conserved and not affected by the thermal I/F.

The laser optical bench is mounted inside the pressurized housing central frame by use of isostatic mounts. The laser is assembled on both sides of the bench, as shown in Figure 17. The lower side carrying the oscillator, while the power amplifier and the frequency conversion is situated at the upper side. In order to realize an efficient and compact space-borne laser, new mounting technologies like soldered optics will be applied. This allows a compact, precise, stable and glue free mounting of the optical laser components. These specially designed mounts have been developed within a DLR funded research project.

Calibration devices: High laser stability and the actual knowledge of the laser frequency are crucial for this kind of instrument; hence, a compact and precise frequency reference is necessary. To minimize the required amount of instrument equipment, it is highly desirable to use the frequency reference not only for absolute frequency monitoring, but also to generate error signals for control of the OPO and the seed lasers. The current approach for MERLIN is to use an air spaced Fizeau interferometer and a CCD detector row for fringe measurements. A verification and also calibration is possible by ground echo calibration (determination of the spectral CH4 line center from scanning its spectral line shape). Preliminary studies show that for the maximum rms deviation of the seed frequencies from their required absolute values, the following values must be reached: the online signal must provide an absolute stability of ~15 MHz rms over lifetime while the offline signal must provide an absolute stability of ~100 MHz rms over the mission lifetime.

An IPDA LIDAR is solely relying on the measurement of relative signal intensities. The intensity of the recorded return signal is a direct function of the emitted laser pulse energy. Even for very good lasers, this variation is in the area of up to 5%. Therefore, it is necessary to monitor the energy of each emitted laser pulse with high accuracy and to correlate with the intensity of the corresponding optical return signals (equation 1).

To generate these internal calibration signals, a small fraction of the emitted laser pulses is extracted and fed directly through an attenuation stage to the instrument detector. Measuring the return and calibration signal with the same detector ensures that variations within the signal chain are eliminated. The challenge here is that because of the very weak return signal (for λon ~1000 photons per shot), the reference signal has to be attenuated by about 14 orders of magnitude.

Receiver: Aside from the laser, the receiver telescope is the most challenging component of MERLIN. Its field of view has to be large enough so that the complete laser ground spot (150 m for 99% of the encircled energy) and additional margin for the satellite and the laser jitter is within this field. The other limiting factor is the size of the detector. For APD (Avalanche Photodiode) detectors, the noise is increasing drastically with the detector area, which means that a detector with a small active area has to be chosen. Furthermore, the overall height of the telescope is strongly limited due to the available space within the launcher fairing (MERLIN baseline: Soyuz ASAP-S inner position). These factors result in a small F-number and thus a complex optical design. Aside from the size, also the mass is a very critical value. To realize such a telescope, serious light-weighting of about 85% has to be applied. Furthermore, the thermal design of the telescope has to be in such a way, that a purely passive thermal control system is sufficient since no extra power for heating is available. The current baseline for MERLIN is a Zerodur off-axis telescope with an aperture of 690 mm and an F-number of 0.65, mounted on an optical bench made of CFRP (Carbon Fiber Reinforced Plastic).

Due to the limited pulse energy and telescope size, the receiver and detector noise are both critical. The detector and the following amplifier stages have to be as noiseless as possible. In the 1650 nm region, InGaAs ADPs seem to be the most promising candidates and are currently the baseline for MERLIN. Also, MCT detectors are very promising candidates. They may offer some interesting features of interest for low light level detection around 1.65 µm due to their low excess noise.

ICU (Instrument Control Unit): The ICU is the sole control instance for the whole instrument and interface between the platform and the payload. Its main tasks include: scheduling of the laser trigger, control of the APD sensor data acquisition and buffering, detector temperature control, payload FDIR (Fault Detection, Isolation and Recovery) support, telecommand decoding and execution, and calculation of the range gate according to the altitude data received from the platform.

Beside the space segment, the other crucial part of the MERLIN mission is its ground segments. The MERLIN mission ground segment is composed of a CGS (Control Ground Segment) and a PLGS (Payload Ground Segment).

Figure 19: MERLIN system architecture (image credit: CNES, DLR)

The ISIS command & control reusable ground segment is based on the ISIS generic operation concept and interfaces definition that will be used by all CNES missions in the future. Its main tasks are to control and monitor the satellite and ensure its health over the mission lifetime. The CNES Earth terminal network is based on stations implemented in Europe, Africa and America. It will be used for commanding and receiving housekeeping data (S-band terminals), and for downloading of the science data (X-band terminals). The ground station network is under improvement on automation and reliability aspects (Ref. 11).

- Main control room (used occasionally, mainly in LEOP and commissioning phases)

- File exchange service.

• A Numerical System Simulator.

Command and Control Center: The CCC will be built on the basis of the ISIS (Initiative for Space Innovative Standards) generic Command and Control Center to be used by all future CNES missions. Its main tasks are to control and monitor the satellite and ensure it remains in good condition throughout the mission lifetime. The CCC is composed of seven specialized subsystems:

• The Automation subsystem manages all the activities and orchestrates CCC operations

• The Monitoring and Control subsystem is responsible for the telemetry and command function and communication with the TT&C ground stations

• The Flight Dynamics subsystem is in charge of flight dynamics calculations linked to attitude and orbit control

• The Data Store subsystem is in charge of storing and managing all the data received and produced throughout the mission's lifetime

• The Mission Operation Support subsystem provides dedicated services to support the mission including on-board software management, alert management, generation of reports and the management of external interfaces

• The Visualization subsystem is the graphical interface dedicated to mission monitoring; it can display real-time telemetry and data available in the Data Store

• The Infrastructure subsystem comprises a set of general services, such as time management, hardware and software management or internal and external communication.

Station network: The CNES multi-mission stations network provides S-band and X-band data services. It will be used for satellite commands and the reception of satellite housekeeping telemetry (S-band), and, in X-band, for downloading science data. The location of the S- and X-band ground stations is shown in Figure 21. Network operations are managed by the Network Operations Center in Toulouse.

PLGS (Payload Ground Segment): In order to process and later on use the collected MERLIN data, an efficient payload ground segment is necessary. The PLGS is responsible for payload operations, the end-to-end handling of MERLIN data and the production, quality control, dissemination and archiving of mission products. It consists of:

- PLMC (Payload Monitoring & Control) which monitors and controls the payload. The PLMC generates commands to control the payload based on inputs from the PLEX, analyses payload behavior, investigates payload anomalies and checks the Level 1a data produced by the PLDP. It provides information to the SPEX (Scientific Products Expertise centers) which can deliver a new version of the Level 1 processor to the PLDP.

• PLDP (Payload Data Processing center) is responsible for processing and delivering science data, that is to say the generation of level 0 to level 3 data and products, and the dissemination of Merlin science data. The PLDP manages the data and product archives

• SPEX (Scientific Products EXpertise centers) dedicated to expert assessments of MERLIN science products. SPEX centers manage the science prototypes to support the development and testing of the reference processors. They deliver new versions of a reference processor to the PLDP and relay the reprocessing campaign required by the Science Advisory Group.

The MERLIN mission will provide the following data products:

• Level 0: for each individual laser shot the range-gated backscatter signal data including the ground returns and data from aerosols and clouds together with auxiliary data such as ranging information, laser pulse energy, time, wavelength in a chronological data sequence for the payload operating in observation mode

• Level 1a: Level 0 data with all calibrations computed and appended, but not applied. The payload data are annotated from universal time, satellite position, velocity and pointing

Various additional products can be derived from the observations of the MERLIN mission. As a secondary product, MERLIN could provide estimates of the vegetation vertical structure, and its temporal dynamics with a view to obtain reliable extra constraints on the location and extent of land use change, especially deforestation and re-/afforestation activities. Estimates of the vegetation vertical structure also have the potential to improve knowledge of terrestrial above ground biomass growth and its dynamics because there is a direct and increasingly better understood statistical relationship to above ground carbon storage.

A further secondary product of the MERLIN mission could be the detection of cloud boundaries, including cloud base for small to moderate cloud optical thickness (≤ 1). MERLIN will be one of a number of anticipated next-generation LIDAR missions that will add to cloud climatology studies. The sensitivity to aerosol backscatter enables detection of elevated aerosol layers associated with major forest fires, volcanic eruptions, and other biomass burning activities on spatial scales that provide plume boundary information.

In-flight radiometric calibration of the LIDAR signals over sites with well-characterized reflectance properties could be convenient for comparison with LIDAR theoretical models in order to assess LIDAR performance. The LIDAR retro-reflectance includes the hot spot enhancement factor that applies to the broader angular reflectance normally observed using passive radiometers or imaging spectrometers.

TOMS (Training, Operation and Maintenance Simulator):

A satellite simulator is aimed at modelling all the operational functions seen from the CCC (Control Command Center) through the housekeeping telemetry (TM) and the telecommand (TC). The use of a Training, Operation and Maintenance Simulator (TOMS) allows (Ref. 16):

• the elaboration and the validation of the CCC

• the technical qualification of the system, including

- the validation of the programming plan (messages sent to the Payload) and the validation of the system global performances

- the qualification and the validation of the on-board/ground interfaces.

• the operational qualification of the system

• the operators' training before and after the launch of the satellite

• the preparation and the validation of exceptional operational procedures after the launch

• the anomaly investigation when a breakdown occurs after the launch.

To achieve those objectives, the TOMS, deployed at the CCC, has to substitute itself to the real satellite and the ground stations, and make the operation teams based at the CCC believe they are communicating with the true satellite.

Architecture and conception:

A TOMS has to react as close as possible to the real satellite. Then it is built around two types of components:

- An efficient simulation platform able to run tests faster than the real time

- A set of numerical models representing each equipment with a high level of representativeness.

At CNES, the TOMS components are all developed around a simulation infrastructure called BASILES (BAncs, SImulateurs et Logiciels d'Etudes de Satellites). But contrary to usual projects which integrate models from one or two suppliers, MERLIN has to undertake a challenge which consists in integrating models from four different suppliers:ADS-F (Airbus Defense and Space-France), Myriade Evolution, ADS-G and CNES.

1) BASILES Infrastructure: BASILES is an open simulation infrastructure for design, development and deployment of complex systems. It allows representing complex systems within discrete event simulation as well as integrating real equipment. Designed at first for numerical simulation of satellites, the BASILES components have been used in several space related missions, for dimensioning space infrastructure, validation of hardware and software components, technical and operational validation of global space systems and their components. BASILES has been designed to maintain continuity across disciplines, projects, phases and subsystems.

But BASILES simulation infrastructure, like all existing proprietary simulators, does not support natively SMP-2 ( Simulation Model Portability version 2) models. A model which is designed for BASILES must obey to strict rules concerning the model structure to be compliant with the BASILES production tools so that it can be compiled, deployed and executed. And, the services provided by the kernel are specific to BASILES and cannot be used by models which only know of SMP-2 interfaces.

To overcome these limitations and to allow importing SMP-2 models and running SMP-2 simulations in the BASILES infrastructure, a software component has been developed to provide adaptation of the SMP-2 interfaces to the BASILES interfaces. The role of this component is to hide SMP-2 specifics to the infrastructure and allow integrating and running any SMP-2 compliant model into the BASILES infrastructure.

2) Avionic Core Numerical Models: The MERLIN satellite development is supported by Airbus Defence and Space France and who then provide the avionics Core (OBC, RIU, STR, MTQ, MAG, CSS) according to the Myriade Evolutions organization. Accordingly, they are in charge of the development, the qualification and the delivery of all the associated numerical models for the TOMS.

ADS-F uses its own simulation platform called SimTG (Simulation Third Generation) to develop and run the SMP-2 models. Most of the avionic core models for MERLIN are adapted from existing models, used on previous projects, which require few adaptations to be compliant with ISIS and SMP-2 standards.

In this context, MERLIN takes benefits of simulation standards which allow the exchange of models running on, on one side, SimTG for central software validation and functional chain validation and, on the other side, BASILES for TOMS uses. Besides to reduce the integration risks on BASILES and secure the TOMS development schedule, ADS-F runs validation tests on both simulation platform using two different validation environments (Java for SimTG and Tcl for BASILES).

On top of that, ADS-F provides all the physical models, the central software and the satellite database. The physical models enter in the TOMS development process to qualify the simulator. The central software is integrated to the TOMS thanks to an emulator which allows to simulate the exact behavior of the satellite in terms of command/control and the satellite database is used to configure the models to avoid inconsistencies between simulation and real life

3) Myriade Evolutions Numerical Models: The Myriade Evolutions project provides nine numerical models which correspond to the common units of the platform (Reaction Wheel, PCDU, GNSS, Mass Memory, Battery, Solar Generator, Propulsion, X-band Emitter, S-band Transceiver). By conception those models are SSRA(Spacecraft Simulator Reference Architecture)/SMP-2 and ISIS compliant. Besides even if they are developed using UMF (Universal Modelling Framework) — SMP-2 conception tools developed by ESA, they are tested and validated on the BASILES infrastructure. The Myriade Evolutions development process ensures low risk integration for the TOMS development.

4) Payload Numerical Models: The MERLIN Payload development is supported by Airbus Defence and Space Germany who then provides the Payload Numerical Model. Based on specifications written by CNES (e.g. interface definition and expected representativeness), they develop the model using SimTG infrastructure as their French counterparts. And to ensure a successful integration into BASILES platform during TOMS development, they execute validation tests on both platform SimTG and BASILES. Besides this model is deployed on the Payload Operational Center on simulation facilities and is used on ADS-F bench to contribute to the central software validation. This model, as the previous ones, takes benefit of an intensive use in different environments to contribute to a high level of maturity when deployed on TOMS.

5) Common Patrimony Numerical Models: CNES simulation department supports the ISIS approach by developing generic physical (Environment, Dynamics, Thermal) models which aims at being integrated on TOMS for MERLIN and all the future missions. Those models have been specifying taking into account the lessons learned and the needs from AOCS and Space mechanics teams to TOMS. They implement the ISIS and SMP-2 standards. Developed using UMF, they are tested on BASILES infrastructure based on data propagation concepts. The will is to have on shelves generic and easily configurable models which can be integrated without difficulty on BASILES to develop a TOMS.

In summary, the MERLIN TOMS takes benefits of the lessons learned acquired on several projects over the past 10 years. It follows the standardization dynamics supported by CNES and its partners through ISIS and by ESA through SSRA and SMP-2. The implementation of those new processes allows to reduce costs, to optimize the development schedules and to contribute to make system more efficient and reliable. MERLIN is proud to be the first project to support and implement the standardization for simulation activities and contribute in this way to the success of the mission.

Besides, the achieved works will lead to provide generic and modular solutions to future projects (SWOT, SVOM, etc.), to optimize the development schedule, to provide efficient validation facilities and to ensure a unique operational interface whatever the mission.

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).